The finding is not likely to surprise bisexuals, who have long asserted that attraction often is not limited to one sex. But for many years the question of bisexuality has bedeviled scientists. A widely publicized study published in 2005, also by researchers at Northwestern, reported that “with respect to sexual arousal and attraction, it remains to be shown that male bisexuality exists.”

That conclusion outraged bisexual men and women, who said it appeared to support a stereotype of bisexual men as closeted homosexuals.

In the new study, published online in the journal Biological Psychology, the researchers relied on more stringent criteria for selecting participants. To improve their chances of finding men aroused by women as well as men, the researchers recruited subjects from online venues specifically catering to bisexuals.

Men in the 2005 study, on the other hand, were recruited through advertisements in gay-oriented and alternative publications and were identified as heterosexual, bisexual or homosexual based on responses to a standard questionnaire.

In both studies, men watched videos of male and female same-sex intimacy while genital sensors monitored their erectile responses. While the first study reported that the bisexuals generally resembled homosexuals in their responses, the new one finds that bisexual men responded to both the male and female videos, while gay and straight men in the study did not.

Both studies also found that bisexuals reported subjective arousal to both sexes, notwithstanding their genital responses. “Someone who is bisexual might say, ‘Well, duh!’” said Allen Rosenthal, the lead author of the new Northwestern study and a doctoral student in psychology at the university. “But this will be validating to a lot of bisexual men who had heard about the earlier work and felt that scientists weren’t getting them.”

The Northwestern study is the second one published this year to report a distinctive pattern of sexual arousal among bisexual men.

In March, a study in Archives of Sexual Behavior reported the results of a different approach to the question. As in the Northwestern study, the researchers showed participants erotic videos of two men and two women and monitored genital as well as subjective arousal. But they also included scenes of a man having sex with both a woman and another man, on the theory that these might appeal to bisexual men.

The researchers — Jerome Cerny, a retired psychology professor at Indiana State University, and Erick Janssen, a senior scientist at the Kinsey Institute — found that bisexual men were more likely than heterosexuals or gay men to experience both genital and subjective arousal while watching these videos.

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Dr. Lisa Diamond, a psychology professor at the University of Utah and an expert on sexual orientation, said that the two new studies, taken together, represented a significant step toward demonstrating that bisexual men do have specific arousal patterns.

“I’ve interviewed a lot of individuals about how invalidating it is when their own family members think they’re confused or going through a stage or in denial,” she said. “These converging lines of evidence, using different methods and stimuli, give us the scientific confidence to say this is something real.”

The new studies are relatively small in size, making it hard to draw generalities, especially since bisexual men may have varying levels of sexual, romantic and emotional attraction to partners of either sex. And of course the studies reveal nothing about patterns of arousal among bisexual women. The Northwestern study included 100 men, closely split among bisexuals, heterosexuals and homosexuals. The study in Archives of Sexual Behavior included 59 participants, among them 13 bisexuals.

The new Northwestern study was financed in part by the American Institute of Bisexuality, a group that promotes research and education regarding bisexuality. Still, advocates expressed mixed feelings about the research.

Jim Larsen, 53, a chairman of the Bisexual Organizing Project, a Minnesota-based advocacy group, said the findings could help bisexuals still struggling to accept themselves.

“It’s great that they’ve come out with affirmation that bisexuality exists,” he said. “Having said that, they’re proving what we in the community already know. It’s insulting. I think it’s unfortunate that anyone doubts an individual who says, ‘This is what I am and who I am.’ ”

“This unfortunately reduces sexuality and relationships to just sexual stimulation,” Ms. Ruthstrom said. “Researchers want to fit bi attraction into a little box — you have to be exactly the same, attracted to men and women, and you’re bisexual. That’s nonsense. What I love is that people express their bisexuality in so many different ways.”

Despite her cautious praise of the new research, Dr. Diamond also noted that the kind of sexual arousal tested in the studies is only one element of sexual orientation and identity. And simply interpreting results about sexual arousal is complicated, because monitoring genital response to erotic images in a laboratory setting cannot replicate an actual human interaction, she added.

“Sexual arousal is a very complicated thing,” she said. “The real phenomenon in day-to-day life is extraordinarily messy and multifactorial.”

A version of this article appears in print on August 23, 2011, on Page D1 of the New York edition with the headline: No Surprise for Bisexual Men: Report Indicates They Exist. Order Reprints|Today's Paper|Subscribe